1. Field of the Invention
This invention relates to a radiation image storage panel using a stimulable phosphor, and more particularly to a radiation image storage panel which provides an image of high sharpness.
2. Description of the Prior Art
As is well known in the art, a photographic method using a silver salt such as radiography has generally been employed to obtain a radiation image. Recently, from the viewpoint of problems as shortage of silver resources, a method of obtaining a radiation image without using a silver salt has been desired.
An example of such a method is disclosed in U.S. Pat. No. 3,859,527. In the method of the patent, is used a radiation image storage panel comprising a stimulable phosphor which emits light when stimulated by an electromagnetic wave selected from visible light and infrared rays after exposure to a radiation (The term "radiation" as used herein means an electromagnetic wave or a corpuscular radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, high-energy neutron rays, cathode rays, vacuum ultraviolet rays, ultraviolet rays, or the like.). The method comprises the steps of (i) causing the stimulable phosphor of the panel to absorb a radiation passing through an object, (ii) scanning the panel with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the panel as instantaneous light emission, and (iii) electrically converting the emitted light to an image.
By the way, it is well known in the art that in the conventional radiography in which an intensifying screen is used in combination with a film, the sharpness of the image obtained depends upon the degree of spread of the instantaneous light emitted by the phosphor in the intensifying screen. In contrast to this, in the above-mentioned method for recording and reproducing a radiation image utilizing the stimulability of a stimulable phosphor, the sharpness of the image obtained does not depend upon the degree of spread of the instantaneous light emitted by the stimulable phosphor in the panel, but depends upon the degree of spread of the stimulating rays in the panel. The reason for this will be explained as follows. That is, in the above-mentioned method for recording and reproducing a radiation image, the radiation image stored in the panel is taken out of the panel sequencially as mentioned above. Therefore, all of the light emission caused by the stimulating rays at a certain period (ti) is desirably detected as the output of a certain picture element (xi, yi) on the panel which is exposed to the stimulating rays during the period (ti). Where the stimulating rays spread in the panel due to scattering or the like and stimulates the phosphor surrounding the picture element (xi, yi) in addition to the picture element (xi, yi), the output for the area broader than the picture element (xi, yi) is detected as the output of the picture element (xi, yi). Accordingly, if the light emission caused by the stimulating rays during the period (ti) is only the one emitted by the picture element (xi, yi) solely which has been exactly exposed to the stimulating rays during the period (ti), the instantaneously emitted light does not affect the sharpness of the image obtained no matter how the emitted light spreads in the panel.
The radiation image storage panel employed in the above-mentioned method for recording and reproducing a radiation image at least has a fluorescent layer comprising a proper binder and a stimulable phosphor particles dispersed therein. Although the fluorescent layer itself can be a radiation image storage panel when the fluorescent layer is self-supporting, the fluorescent layer is generally provided on a proper substrate to form a radiation image storage panel. Further, a protective layer for physically and chemically protecting the fluorescent layer is usually provided on the exposed surface of the fluorescent layer. In the conventional radiation image storage panel having such a structure, the mean free path of the stimulating rays in the fluorescent layer becomes relatively long due to scattering or the like, and accordingly the stimulating rays broadly spread therein. Therefore, an image of high sharpness cannot be obtained by the conventional radiation image storage panel.